On the Trinity (De Trinitate) is a Latin book written by Augustine of Hippo to discuss the Trinity in context of the logos. It is placed by him in his Retractations among the works written (meaning begun) in AD 400. In letters of AD 410, AD 414, and at the end of AD 415, it is referred to as still unfinished and unpublished. But a letter of A.D. 412 states that friends were at that time asking to complete and publish it. And the letter to Aurelius, which was sent with the treatise itself when actually completed, states that a portion of it, while it was still unrevised and incomplete, was in fact surreptitiously made public. It was still in hand in AD 416: in Book XIII, a quotation occurs from the 12th Book of the De Civitate Dei; and another quotation in Book XV., from the 90th lecture on St. John. The Retractations, which refer to it, are usually dated not later than AD 428. The letter to Bishop Aurelius also states that the work was many years in progress, and was begun in St. Augustine's early manhood, and finished in his old age. Arthur West Haddan inferred from this evidence that it was written between AD 400, when he was forty-six years old, and had been Bishop of Hippo about four years, and AD 428 at the latest; but probably was published ten or twelve years before this date, around 417. It is also the title of works written by at least two other luminaries of the early church: Hilary of Poitiers (the Hammer of the Arians) and Richard of St. Victor.
